Is there anything I should know about the RH Hood Method? Is there anything that would distinguish other RH probes from it? Not drilling could be useful at times for me.
Anyone's opinions, facts, or experience would be nice.
The ASTM F2420 hood test is similar to the calcium chloride testing. The values of the test are in percentage rether than pounds. In both cases it measures the moisture vapor movement and not the internal moisture content.
04-29-2009, 11:27 PM
(This post was last modified: 04-29-2009, 11:30 PM by Trever Shiplee.)
I agree with you Ray the hood method ASTM F2420 is a similar measurement to the calcium chloride method yet the hood method is measuring the amount of free water in the slab while the calcium chloride test is measuring moisture vapor both of these test reflect a 1/4" to 1/2" of the slabs thickness, ASTM F2170 measures at 40% of the slab depth. Our testing lab has performed all three of these test and have not found a true correlation between the hood method ASTM F2420 and the in situ probe method ASTM F2170 we have found though at ideal conditions the hood method has been five to eight percent lower than the in situ probe. In closing when doing an investigation in flooring failure we chose ASTM F2170 due to the depth the test measures and to the lower chance of tester’s error during placement.
Ray..all due respect..r.h. probes cannot measure the internal volume of water..the exact porosity of the concrete has to be known first..which is a virtual impossibility. The most concerning factor is dewpoint, and the hood tests are, in my opinion, an overly complicated method of trying to figure it out.
Moisture meters are sufficient for that and much easier to understand. The hood again will tell you what the r.h. under the hood is..but..so what? If the pores are one gallon size as opposed to quart size (just to give an easy to illustrate example)..and the the amount of moisture measured under the hood is equal, would be much more saturated with smaller pores..a pint of water, the quart size pores are half full, a pint of water, the gallon size are only an 1/8th full..HUGE difference for the same amount of water volume..yet the humidity readings would NOT give an indication of this critical volume difference or it's relative effect on the concrete and the subsequent floor installation. In other words, there is no magic bullet...
Howdy all, I just found this site, so it may take a while to get to know everyone.
The hood method does not replace in-situ probes in any way. The hood will measure vapor being released from the slab in a manner similar to the CaCl test with one important difference. The Calcium Chloride can actually draw moisture from the slab, as proven by CTL labs when they placed a CaCl test on a stable slab (not emitting moisture at all) and the CaCl test showed 4 lbs emissions.
In-situ probes tell us the potential a slab has. Think of a slab that is extremely wet, perhaps wet cured for 4 months, then quickly dried with dehydrators, fans and heat. The top of the slab will dry rather quickly, and you could see great MVER readings from the top 1/4" - 1/2", but the amount of moisture available in the slab could be 98%.
CaCl dome tests will stabilize humidity to between 20-40% dependent upon the ambient temperatures..within the protocol in ASTM F1869...the humidity under the dome attempts to stabilize at 40%, so any moisture content past 40% as free water WILL get absorbed.
No matter what kind of R.H. is used, unless the VOLUME of pore space is known...the volume of water is unknowable. The relative humidity change dramatically with temperature..even if the volume of water does NOT change...so a floor with a "safe" level within the temperature parameters set within the stated standard of F1869...could have a safe r.h. of 65% at 80oF but show over 95% r.h. at 65 degrees..even if the VOLUME of water remains unchanged.
I understand your thinking, but my testing has shown that temperature has less impact than you would think.
Initially we tried to incorporate the psychrometric chart when considering concrete moisture and slab temperature, but extensive field testing shows this is not accurate.
We are working on a chart now that will allow a tester to take a relative humidity test at a slab temperature of 50 degrees and extrapolate where the RH will be at 70 degrees.
As far as water volume goes, I do not agree that one need be concerned about the quantity of free water in the concrete pores unless you are attempting to predict how long a slab will take to dry. A humidity probe placed at the top of a sealed cylinder containing an inch of water in the bottom will read the same as if there were 3 inches of water in the cylinder.